First Report of Powdery Mildew on Sophora flavescens Caused by Erysiphe diffusa in China.

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Sophora flavescens (Fabaceae) is a deciduous subshrub which has been used in Chinese popular medicine for a long history (He et al. 2015). In June 2023, severe powdery mildew symptoms were observed on wild S. flavescens plants on Longwen hill of Guizhou Normal University, Guiyang, China. The incidence was approximately 80% among 100 S. flavescens plants observed. Almost all leaves were infected. Mycelia occurred on both adaxial and abaxial leaf surfaces, petioles, and stems, forming small-to-large patches. Hyphae were hyaline, 5 to 7 μm wide. Hyphal appressoria were solitary. Conidiophores were erect, straight to somewhat flexuous, and 45 to 120 µm long (n = 50). Foot cells were subcylindrical to slightly curved, followed by 2 to 3 shorter cells. Conidia formed singly, were ovoid to cylindrical, 26 to 42 × 12 to18 µm (n = 50). Based on these morphological characteristics, the powdery mildew fungus was tentatively identified as Erysiphe diffusa (Braun and Cook 2012). To confirm the identification, the ribosomal DNA internal transcribed spacer (ITS) and the ribosomal large subunit (LSU) region were amplified and sequenced using primer pairs ITS1/ITS4 (White et al. 1990) and NL1/NL4 (Ziemiecki et al. 1990), respectively. The obtained 647-bp ITS sequence (GenBank accession no. PP130131) displayed 100% identity with the ITS sequences of E. diffusa. The obtained 618-bp LSU sequence (GenBank accession no. PP693303) displayed 100% identity with the ITS sequences of E. diffusa (MT325922 and MT628019) and E. manihoticola (MT106658 and MT106660). Using a phylogenetic tree based on the combined ITS-LSU data, the isolate was grouped in a clade with the E. diffusa strain (GenBank accession no. LC777871). To fulfill Koch's postulates, leaves of three healthy potted S. flavescens plants were inoculated by gently pressing with diseased leaves. Non-inoculated plants were used as controls. All plants were incubated in a greenhouse at 25 ± 2°C, 80% relative humidity. After 15 days, typical powdery mildew symptoms were observed on the inoculated plants, whereas no symptoms were found on the control plants. The reisolated fungus from the inoculated S. flavescens was morphologically identical to that on naturally diseased plants, and the ITS sequence of the reisolated fungus showed 100% identity with PP130131. As the causal fungus of soybean powdery mildew, E. diffusa is known to infect papaya and other legumes, including Lens culinaris and Mimosa caesalpiniifolia (Attanayake et al. 2009; Luz et al. 2019). Particularly, E. diffusa has been previously reported to infect S. flavescens in the United Kingdom (Jones and Baker 2007; Bradshaw et al. 2023), but this is the first report of S. flavescens powdery mildew caused by E. diffusa in China. This work further expands the geographical range of E. diffusa-infected S. flavescens plants.